1. Field of the Invention
The present invention relates generally to electrodynamic, moving voice coil actuators capable of converting energy between electrical and mechanical form and, more particularly, to a diaphragm loudspeaker and momentum or inertial type voice coil actuators that utilize a high energy axially polarized biasing magnet and a multi-component suspension for alignment of the moving coil.
2. Background of Invention
Loudspeakers and momentum type transducers historically have utilized two basic electrodynamic structures based upon a magnetic circuit described in U.S. Pat. No. 2,698,917 (A. T. Van Urk, et al) which describes the use of a ferromagnet having a substantially flat, thin permanent magnet, where the smallest dimension of the magnet is parallel with the direction of magnetization. Of the two most basic descriptions of the substantially flat magnet, the most common is the use of an annular magnet adjacent to a bottom plate with a center post to form one magnetic pole and a top plate with a central hole, creating an annular air gap with the center post to form the second magnetic pole. The second basic description is a disk shaped magnet without a central aperture that has a first pole defined as a top plate having the same as or larger diameter than the magnet, and a second pole formed by a pot type structure where the magnet is centrally aligned with the pot and an annular airgap is formed between the upper edge of the pot and the magnetic top plate.
The annular magnet type electrodynamic motor structure has found a very wide use because the magnet material is inexpensive, and because of the fact that assembly and magnetization are simple to accomplish. However, this design has significant drawbacks. The magnetic leakage flux at the outer edge of the magnetic assembly is strong. When this structure is placed near a CRT or Plasma type video display, the display equipment is degraded. Further, the low magnetic flux output of the ferromagnetic material requires substantial cross-sectional area of the magnet system (transversely to the axis of symmetry). The resulting requisite large physical dimensions are problematic for many new product design considerations.
The pot type magnetic structure gained significant commercial viability with the introduction of rare earth magnets, primarily those containing Neodymium, Iron and Boron. U.S. Pat. No. 5,390,257 (Oslac, et al) describes a system which is based on an axially magnetized, coin or disk-shaped magnet, usually of NdFeB material. The high flux capacity of the NdFeB magnet enables reasonable efficiency with the magnet contained within the voice coil dimension. However, since the area of the magnet is limited by the coil diameter, it is also limited how large magnetic flux can be obtained. Further, it is common to add an axial hole centrally through the assembly to obtain ventilation and this addition will reduce the magnetic flux, overall efficiency, and bandwidth. On the positive side, the system has a moderate depth and cross-sectional area in relation to the coil diameter, something which is very advantageous in some applications.
As is well known in the art, the force generated by an electrodynamic transducer is a product of the current, I, length of coil wire, L and flux density, B so that F=iL{circle around (x)}B. The length of the coil wire that is within the annular magnetic gap is defined as the length, L. This force is what creates the movement of the coil and subsequently generates sound. Building on this concept, inertial voice coil actuators have been used to acoustically stimulate semi-rigid structures to radiate sound. In this application, voice coil actuators have been attached to structures that are relatively large to act as a soundboard e.g. a wall in a room. The wall of the room, when acoustically driven, radiates sound. As is well known in the art, the force generated by an electrodynamic transducer is a product of the current, I, length of coil wire, L and flux density, B so that F=iL{circle around (x)}B. The length of the coil wire that is within the annular magnetic gap is defined as the length, L. This force is what creates the movement of the coil and subsequently generates sound.
A number of inventions for voice coil actuators have been patented among them U.S. Pat. No. 2,341,275 to Holland for Sound Reproducing Instrument; U.S. Pat. No. 3,609,253 for Loudspeaker with Improved Voice Coil Suspension; U.S. Pat. No. 3,728,497 to Komatsu for Dynamic Loudspeaker Using Wall as Diaphragm; U.S. Pat. No. 4,297,537 to Babb for Dynamic Loudspeaker; U.S. Pat. No. 4,951,270 for Audio Transducer Apparatus; U.S. Pat. No. 5,335,284 to Lemons for Coneless, No-Moving-Parts Speaker; and U.S. Pat. No. 5,473,700 Fenner, Jr. for High Gain Transducer and U.S. Pat. No. 3,524,027 to Thurston, et al.
It is known to employ a compliant suspension structure formed in the base wall of a momentum type transducer that is in closest proximity to the soundboard. At the centerline of the base wall is either a threaded insert or protruding threaded bolt. The momentum transducer is then cantilevered off a threaded rod which is mechanically attached to the soundboard. Two problems arise with this design; it significantly increases the transducer's standoff height and profile and, further, forces acting normal to the protruding threaded element are amplified such that they may cause structural failure in the mechanical attachment.
In practice, the annular magnet, magnetizable plates, external housing and structural attachment point as presently known in the art, comprise a system that is large and heavy relative to the total dynamic force the actuator is capable of generating. If the external housing is mounted on a vertical facing surface such as a wall, large bending moments will be placed on the structural attachment point which may be translated to the coil. In sum, the present state of the art provides electrodynamic transducers that are plagued with well known problems of low power handling, limited frequency response, high levels of sound distortion, substantial size and mass, mechanical complexity and high production costs.
Recent innovations include magnetic materials that have produced magnets with substantially greater magnetic energy than ceramic magnets. These magnets have necessitated the redesign of the magnetic circuit to take advantage of the higher magnetizing flux while reducing the volume of the magnet material consumed, thus reducing its size while simultaneously increasing its force density per unit volume. However, these prior art voice coil actuators are not typically designed with suspension systems adequate for actuators driving relatively large structures such as walls and their application in those contexts results in some of the same short falls as was previously known, especially relative to sound quality and distortions.